The present invention relates to a method for manufacturing a semiconductor device having a gallium nitride semiconductor layer.
In the field of semiconductor device such as a light-emitting diode and a semiconductor laser diode, attention has been called to a wide-gap semiconductor device capable of emitting blue light. Such a semiconductor device has a semiconductor layer in which a group 2 element, such as magnesium (Mg) and zinc (Zn), is added to a single crystal of nitride semiconductor which is expressed, for example as (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y N, (0.ltoreq.x.ltoreq.1.0, 0.ltoreq.y.ltoreq.1.0).
The epitaxial growth of the above-described nitride semiconductor is usually carried out by the metal organic chemical vapor deposition (MOCVD) method. However, the semiconductor layer formed by the MOCVD method and applied with the group 2 element such as magnesium and zinc has extremely high resistance in the as-grown state, so that the electric current cannot flow through the layer in order to emit the blue light from the semiconductor device.
There has recently been reported a method for converting the high resistant nitride semiconductor into a p-type semiconductor by applying a special treatment thereto. For example, a low accelerated electron beam is radiated onto the nitride semiconductor to form a low resistance p-type semiconductor (H. Amano et al.,: Jpn. J. Appl. Phys. Vol. 28, 1989, pp. L2112-2114), or the nitride semiconductor is subjected to a heat treatment of 800.degree. C. in a nitrogen atmosphere under the atmospheric pressure or higher For about 20 minutes to also form a low resistance p-type semiconductor (S. Nakamura et al.,: Jpn. J. Appl. Phys. Vol. 31, 1992, pp. L139-142).
The low accelerated electron beam method provides a p-type semiconductor having an extremely high hole density at room temperature in the order of 10.sup.18 cm.sup.-3. However, the depth of the semiconductor which can be treated is limited to the depth wherein the electron beam can be permeated, and therefore about 0.3 m in the case of electron beam applied at accelerated voltage of 6 to 30 KV (S. Nakamura et al.,: Jpn. J. Appl. Phys. Vol. 31, 1992, pp. L139-142). In addition, since the treatment must be executed by scanning the electron beam in vacuum, not only the apparatus for the treatment must be enlarged, but also requires a longer time to treat one wafer, thereby causing a drawback for the mass production.
On the other hand, the heat treatment imposes less restriction than the low accelerated electron beam radiation method with regard to the treating depth, and more over, is appropriate for mass production, since a large number of wafers can be put in a heating furnace at one charge. However, as can be understood from the experiments conducted by S. Nakamura et al., the discoverers of the phenomenon, the hole density at room temperature remains at 3.times.10.sup.17 cm.sup.-3, which is clearly smaller than that achieved by the low accelerated electron beam treatment. The value 3.times.10.sup.17 cm.sup.-3 is sufficient for forming the pn diode which is the basic component of the light-emitting diode and semiconductor laser, so that the heat treatment has heretofore been employed in practice for manufacturing the light-emitting diode.
One of the problems which is generated by the heat treatment when manufacturing the semiconductor devices and more particularly, semiconductor laser devices is the contact resistance of electrodes.
In general, at the interface between a metal and a semiconductor, there is formed an electrical barrier which restricts the transmission of the electrons and the holes. When the electric barrier is high, the electric current between the metal and the semiconductor is rectified so as to be imparted with a threshold voltage when electric current is applied. Such a junction, which is called a Schottky junction, is extremely disadvantageous except when intentionally used, so that there is usually taken some measures to render the contact between the metal and the semiconductor an ohmic contact, and hence, linear.
There are several ways for providing the ohmic contact, the basic necessary condition being sufficiently increasing the density of the carriers adjacent the interface, that is, electrons in the n-type semiconductor and holes in p-type semiconductor. When the density of the carriers directly under the electrode is sufficiently high, and the Fermi level is within the conduction band or the valence band, the semiconductor becomes so-called metallic, thereby providing the ohmic contact irrespective of the kind of metal used as the electrode.
However, in the above-described heat treatment for imparting a p-type characteristic, the hole density is not sufficient so that the problem of the large contact resistance at the electrode still remains.